In chapter 10, the children take their air rifles outside to shoot and Atticus encourages them to aim at tin cans or blue jays. Atticus then tells Scout and Jem that it is a sin to kill a mockingbird. When Scout asks Miss Maudie why Atticus said it was a sin to kill a mockingbird, Maudie tells her,
Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird (Lee, 93).
Essentially, mockingbirds are peaceful, defenseless birds that do not harm anyone and only spread joy, which is why people should protect them and refrain from harming them. In the novel, mockingbirds symbolically represent vulnerable, innocent individuals like Tom Robinson and Arthur "Boo" Radley, who become the unfortunate victims of the town's prejudice. Later on, Scout metaphorically applies Atticus's comments regarding mockingbirds to Boo Radley's situation after Bob Ewell's vicious attack. She likens Boo Radley to an innocent, defenseless mockingbird and demonstrates her maturity and integrity by agreeing with Sheriff Tate's decision to not tell the town about Boo's heroics in order to protect him.